In my personal opinion, always, always, make a backup. Of your site, of your custom theme, and of your content. It's the cardinal rule of code, college papers, and anything else they'll let you have more than one of in life. We have tools for all three, and here's how to put them to use:
Copying a site is a fast and painless process, being a few clicks at most, and gives you a complete copy of the content and details. There are a few limitations to it, like blog content and products. You'll also want to save your theme progress separately.
Backing up your theme is even easier. Click the Export Theme button on the bottom left of the code editor, and you'll be able to save your code work. If you're making changes to the theme, always do this often. The only thing worse than losing progress on code is having a theme that doesn't work, when you're not sure how and don't have a backup to revert to.
Archiving your content is a lot like copying your site, except you get an offline-accessible ZIP file of the content. It's not as simple as a site copy, but you can take it anywhere you like.
Always keep these in mind when making changes to an established site. Even if you have an easy way to fix it up, play it safe. Even failsafes can fail. We have an automatic save system that's great for making sure you don't lose progress, but it won't know if you meant to replace your entire product page with your album of cute puppy pictures or not, and there is no "undo" button.
Sorry for the loss, @bobafett. Hopefully we can help make sure nobody here has to encounter that kind of thing again.
